Why does my Flutter app crash at startup on iOS?

Ok, so here's what happened. Originally the app crashed on iOS because there were issues with the GoogleServices-info.plist file (probably). There were no error messages, so I didn't know where to look. I finally just went back to the simple demo app and started layering things back in to isolate the issue. The demo app would build and run on iOS with no problems, but when I added the Firebase dependencies (as a group), it failed. I had added the GoogleServices-info.plist file manually (outside of Xcode) and adjusted the references in ios/Runner.xcodeproj/project.pbxproj using a text editor, but that didn't do the trick.

At this point I finally broke down and got a MacBook. Debugging on the Mac, I could now see that the app crashed in debug mode on the emulator as well as when in release mode on a device, but still no helpful error messages. I added the GoogleServices-info.plist file using Xcode, so at least that piece was correct, but the app still crashed immediately. Going through the documentation, I realized that when I made the sample app, I had forgotten to add the app ID in the info.plist file for AdMob. At this point it was just AdMob causing the crash, not the other Firebase dependencies. I added that back in and things ran fine. I wish I had looked at that a month ago. Ugh.

Thanks @Mikhail Tokharev and @season for taking a stab at such a vague question. I wish I had more detail to share initially, but I didn't know enough yet to know what could be going wrong.

TL;DR

1. Just get a Mac. Trying to debug through Codemagic won't work.
2. For Firebase, be sure you actually add your GoogleServices-info.plist file through Xcode. Trying to add it manually and hack the project.pbxproj file probably won't work (this a practical recommendation, I didn't prove this conclusively). https://firebase.google.com/docs/flutter/setup?platform=ios#add-config-file
3. For AdMob, be sure to add your application ID to the info.plist. https://pub.dev/packages/firebase_admob

I've been facing this error since the last few days and I happen to solve it today. So, I would like to share it with anyone facing startup error after connecting their iOS app to firebase.

I had followed all the steps while setting up a new iOS app with firebase and this was what exactly caused the error, in my case. If you have placed import 'Firebase' and Firebase.configure() in your AppDelegate.swift, then remove it and run your app again. I removed those two lines and it worked fine. Test it with Cloud Firestore (just make a button write something to your cloud firestore) and make sure you add await Firebase.initializeApp(); as well when the button is pressed.

I did all of those steps and it worked fine with me.

P.S. I did it with android studio in a Mac.